Conventionally, in the case of producing silicon wafers from a silicon single crystal ingot, for example, a single crystal ingot grown by Czochralski method (CZ method) is cylindrically ground to be finished to have a predetermined size (diameter), and its top and tale which are unusable as products are cut off. Then, the ingot is cut at a predetermined position and to be blocks having lengths for being possible to be put in a slicing apparatus such as an inner diameter blade or a wire saw. In this case, samples for quality inspection such as for oxygen concentration are sliced at the same time. Next, silicon wafers can be obtained by slicing each of the blocks by a predetermined thickness.
As a method for producing a semiconductor ingot such as a silicon single crystal, Floating Zone method (FZ method) and so forth are known as well as CZ method. However, in recent years, diameter of semiconductor wafers has enlarged, for example, CZ method is used in the case of growing a silicon single crystal ingot having a diameter measured up to 300 mm or more.
In the case of growing a silicon single crystal ingot by CZ method, a quartz crucible is used as a container containing a silicon melt to be a raw material. However, in the growth, oxygen solves in the melt from a quartz crucible and oxygen is taken in the grown ingot. In addition, in the case of obtaining a single crystal having low oxygen concentration, MCZ method in which a raw material melt is applied with magnetic field and thereby convection is suppressed is occasionally adopted.
Oxygen concentration in the single crystal is controlled by adjusting rotation rate of the crucible, pulling rate, temperature gradient, and so forth. However, it is inevitable that an extent of dispersion is caused inside the same crystal. However, oxygen concentration in the crystal has an effect on gettering capability and such after the crystal is processed into wafers, and has an effect on yield in the device process. Therefore, oxygen concentration in the wafer is required to be in a predetermined standard range and it is required to evaluate oxygen concentration of the grown ingot.
Conventionally, as a method for evaluating quality of a grown silicon single crystal ingot, there is a method for evaluating parts of both ends of each of the blocks after an ingot is cut into blocks having a predetermined length.
For example, when an ingot is cut into blocks, a cut position in the ingot is made to be a discretionary position or a position determined according to information such as pulling rate. And, after the cutting, slab samples for measuring quality are obtained from both ends of each of the blocks, and admission decision of the block is performed by evaluating resistivity or oxygen concentration of the slab samples.
According to such a method, resistivity inside the block can be very accurately estimated by calculation from segregation coefficient and such. However, oxygen concentration inside the block is often higher or lower than oxygen concentration at an end face of the block. In particular, if a single crystal is grown according to MCZ method, oxygen concentration is occasionally changed unexpectedly according to distribution of the magnetic field or the like.
Therefore, in the case that standard of oxygen concentration is tight, in such a crystal as having a peak of oxygen concentration in the block, even when oxygen concentration at end faces of the block is admitted, oxygen concentration in the block does not always satisfy the standard. In particular, a wafer obtained by slicing a block in which oxygen concentrations at the end faces are admitted narrowly to the standard has possibility of causing a problem at a device process due to oxygen concentration fault.
Moreover, in such a method for evaluating oxygen concentration only at end faces of blocks as described above, a local shift in oxygen concentration is frequently overlooked. Although there is a problem in the single crystal production recipe, it cannot be occasionally recognized. Therefore, there is also a problem that it is difficult to improve the single crystal production recipe.
As a method for improving such problems, there has been suggested a method for measuring an oxygen concentration distribution in the longitudinal direction (growth axis direction) of an ingot according to an infrared absorption spectrometer or the like and then cutting only parts fitting in a predetermined oxygen concentration as blocks for slicing (Japanese Patent Application Laid-open (kokai) No. 2002-174593). Specifically, after a single crystal ingot is cylindrically ground, infrared is irradiated from a radial direction and oxygen concentration is measured from the absorption. Such measurement is performed by predetermined intervals along the longitudinal direction of the ingot and thereby an oxygen concentration distribution in the longitudinal direction can be measured.
However, oxygen concentration in a horizontal direction (radial direction) to be measured in the ingot state as described above is average in the part, and therefore, for example, in the case that there is difference in oxygen concentration between the center and the periphery in the ingot, even when the measurement value satisfies with a standard, oxygen concentration in the center or the periphery does not actually satisfy the standard, occasionally. That is, in such a method as described above, it is feared that a block that does not satisfy the standard of oxygen concentration in the center or the periphery thereof is determined as acceptance.
Accordingly, there are problems that if a block determined as acceptance is processed into wafers, it is highly possible to put in a device process a wafer that does not actually satisfy a standard of oxygen concentration, and reliability is not sufficient.